Wavelength conversion component with a diffusing layer

Wavelength conversion component with a diffusing layer

A light emitting device comprises at least one solid-state light source (LED) operable to generate excitation light and a wavelength conversion component located remotely to the at least one source and operable to convert at least a portion of the excitation light to light of a different wavelength. The wavelength conversion component comprises a light transmissive substrate having a wavelength conversion layer comprising particles of at least one photoluminescence material and a light diffusing layer comprising particles of a light diffractive material. This approach of using the light diffusing layer in combination with the wavelength conversion layer solves the problem of variations or non-uniformities in the color of emitted light with emission angle. In addition, the color appearance of the lighting apparatus in its OFF state can be improved by implementing the light diffusing layer in combination with the wavelength conversion layer. Moreover, significant reductions can be achieved in the amount phosphor materials required to implement phosphor-based LED devices.Related Terms:PhosphorLed DeviceLightingLighting Apparatus

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No. 13/273,212, entitled “Wavelength Conversion Component With A Diffusing Layer”, filed Oct. 13, 2011, now issued as U.S. Pat. No. 8,604,678, which claims the benefit of priority to U.S. Provisional Patent Application Ser. No. 61/427,411, entitled “Solid-State Light Emitting Devices with Remote Phosphor Wavelength Conversion Component”, filed Dec. 27, 2010, which are hereby incorporated by reference in their entireties. U.S. application Ser. No. 13/273,212 is also a continuation-in-part of U.S. application Ser. No. 13/253,031, entitled “Solid-State Light Emitting Devices and Signage with Photoluminescence Wavelength Conversion,” filed on Oct. 4, 2011, now issued as U.S. Pat. No. 8,610,340, which claims the benefit of U.S. Provisional Patent Application Ser. No. 61/390,091, entitled “Solid-State Light Emitting Devices and Signage with Photoluminescence Wavelength Conversion,” filed on Oct. 5, 2010, which are hereby incorporated by reference in their entireties.

FIELD

This disclosure relates to solid-state light emitting devices that use a remotely positioned phosphor wavelength conversion component to generate a desired color of light.

BACKGROUND

White light emitting LEDs (“white LEDs”) are known and are a relatively recent innovation. It was not until LEDs emitting in the blue/ultraviolet part of the electromagnetic spectrum were developed that it became practical to develop white light sources based on LEDs. As taught, for example in U.S. Pat. No. 5,998,925, white LEDs include one or more one or more photoluminescent materials (e.g., phosphor materials), which absorb a portion of the radiation emitted by the LED and re-emit light of a different color (wavelength). Typically, the LED chip or die generates blue light and the phosphor(s) absorbs a percentage of the blue light and re-emits yellow light or a combination of green and red light, green and yellow light, green and orange or yellow and red light. The portion of the blue light generated by the LED that is not absorbed by the phosphor material combined with the light emitted by the phosphor provides light which appears to the eye as being nearly white in color. Alternatively, the LED chip or die may generate ultraviolet (UV) light, in which phosphor(s) to absorb the UV light to re-emit a combination of different colors of photoluminescent light that appear white to the human eye.

Due to their long operating life expectancy (>50,000 hours) and high luminous efficacy (70 lumens per watt and higher) high brightness white LEDs are increasingly being used to replace conventional fluorescent, compact fluorescent and incandescent light sources.

Typically the phosphor material is mixed with light transmissive materials, such as silicone or epoxy material, and the mixture applied to the light emitting surface of the LED die. It is also known to provide the phosphor material as a layer on, or incorporate the phosphor material within, an optical component, a phosphor wavelength conversion component, that is located remotely to the LED die (“remote phosphor” LED devices).

One issue with remote phosphor devices is the non-white color appearance of the device in its OFF state. During the ON state of the LED device, the LED chip or die generates blue light and the phosphor(s) absorbs a percentage of the blue light and re-emits yellow light or a combination of green and red light, green and yellow light, green and orange, or yellow and red light. The portion of the blue light generated by the LED that is not absorbed by the phosphor combined with the light emitted by the phosphor provides light which appears to the human eye as being nearly white in color. However, for a remote phosphor device in its OFF state, the absence of the blue light that would otherwise be produced by the LED in the ON state causes the device to have a yellowish, yellow-orange, or orange-color appearance. A potential consumer or purchaser of such devices that is seeking a white-appearing light may be quite confused by the yellowish, yellow-orange, or orange-color appearance of such devices in the marketplace, since the device on a store shelf is in its OFF state. This may be off-putting or undesirable to the potential purchasers and hence cause loss of sales to target customers.

Another problem with remote phosphor devices can be the variation in color of emitted light with emission angle. In particular, such devices are subject to perceptible non-uniformity in color when viewed from different angles. Such visually distinctive color differences are unacceptable for many commercial uses, particularly for the high-end lighting that often employ LED lighting devices.

Yet another problem with using phosphor materials is that they are relatively costly, and hence correspond to a significant portion of the costs for producing phosphor-based LED devices. For a non-remote phosphor device, the phosphor material in a LED light is typically mixed with a light transmissive material such as a silicone or epoxy material and the mixture directly applied to the light emitting surface of the LED die. This results in a relatively small layer of phosphor materials placed directly on the LED die, that is nevertheless still costly to produce in part because of the significant costs of the phosphor materials. A remote phosphor device typically uses a much larger layer of phosphor materials as compared to the non-remote phosphor device. Because of its larger size, a much greater amount of phosphor is normally required to manufacture such remote phosphor LED devices. As a result, the costs are correspondingly greater as well to provide the increased amount of phosphor materials needed for such remote phosphor LED devices.

Therefore, there is a need for improved approaches to implement LED lighting apparatuses that maintains the desired color properties of the devices, but without requiring the large quantities of photoluminescent materials (e.g. phosphor materials) that are required in the prior approaches. In addition, there is a need for an improved approach to implement LED lighting apparatuses which addresses perceptible variations in color of emitted light with emission angle, and which also addresses the non-white color appearance of the LED lighting apparatuses while in an OFF state.

SUMMARY

Embodiments of the invention concern light emitting devices comprising one or more solid-state light sources, typically LEDs, that are operable to generate excitation radiation (typically blue light) and a remote wavelength conversion component, containing one or more excitable photoluminescence materials (e.g., phosphor materials), that is operable to convert at least a portion of the excitation radiation to light of a different wavelength. When using a blue light radiation source, the emission product of the device comprises the combined light generated by the source and the wavelength conversion component and is typically configured to appear white in color. When using an UV source, the wavelength conversion component(s) may include a blue wavelength conversion component and a yellow wavelength conversion component with the outputs of these components combining to form the emission product. The wavelength conversion component comprises a light transmissive substrate such as a polymer or glass having a wavelength conversion layer comprising particles of the excitable photoluminescence material (such as phosphor) and a light diffusing layer comprising particles of a light diffractive material (such as titanium dioxide). In accordance with some embodiments of the invention, the wavelength conversion and light diffusing layers are in direct contact with each other and are preferably deposited by screen printing or slot die coating. As used herein, “direct contact” means that there are no intervening layers or air gaps.

One benefit of this approach is that by selecting an appropriate particle size and concentration per unit area of the light diffractive material, an improvement is obtained in the white color appearance of a LED device in its OFF state. Another benefit is an improvement to the color uniformity of emitted light from an LED device for emission angles over a ±60° range from the emission axis. Moreover the use of a light diffusing layer having an appropriate particle size and concentration per unit area of the light diffractive material can substantially reduce the quantity of phosphor material required to generate a selected color of emitted light, since the light diffusing layer increases the probability that a photon will result in the generation of photoluminescence light by directing light back into the wavelength conversion layer. Therefore, inclusion of a diffusing layer in direct contact with the wavelength conversion layer can reduce the quantity of phosphor material required to generate a given color emission product, e.g., by up to 40%. In one embodiment the particle size of the light diffractive material is selected such that excitation radiation generated by the source is scattered more than light generated by the one or more phosphor materials.

According to some embodiments of the invention a wavelength conversion component for a light emitting device comprising at least one light emitting solid-state radiation source, comprises a light transmissive substrate having a wavelength conversion layer comprising particles of at least one photoluminescence material and a light diffusing layer comprising particles of a light diffractive material; and wherein the layers are in direct contact with each other. Preferably the wavelength conversion layer comprises a mixture of at least one phosphor material and a light transmissive binder while the light diffusing layer comprises a mixture of the light diffractive material and a light transmissive binder. To minimize optical losses at the interface of the layers it is preferred that the layers comprise the same transmissive binder. The binder can comprise a curable liquid polymer such as a polymer resin, a monomer resin, an acrylic, an epoxy, a silicone or a fluorinated polymer. The binder is preferably UV or thermally curable.

To reduce the variation in emitted light color with emission angle the weight loading of light diffractive material to binder is in a range 7% to 35% and more preferably in a range 10% to 20%. The wavelength conversion and light diffusing layers are preferably deposited by screen printing though they can be deposited using other deposition techniques such as spin coating or doctor blading. The light diffractive material preferably comprises titanium dioxide (TiO2) though it can comprise other materials such as barium sulfate (BaSO4), magnesium oxide (MgO), silicon dioxide (SiO2) or aluminum oxide (Al2O3).

In one arrangement the light diffractive material has an average particle size in a range 1 μm to 50 μm and more preferably in a range 10 μm to 20 μm. In other arrangements the light diffractive material has a particle size that is selected such that the particles will scatter excitation radiation relatively more than they will scatter light generated by the at least one photoluminescence material. For example, for blue light radiation sources, the light diffractive particle size can be selected such that the particles will scatter blue light relatively at least twice as much as they will scatter light generated by the at least one phosphor material. Such a light diffusing layer ensures that a higher proportion of the blue light emitted from the wavelength conversion layer will be scattered and directed by the light diffractive material back into the wavelength conversion layer increasing the probability of the photon interacting with a phosphor material particle and resulting in the generation of photoluminescent light. At the same time, phosphor generated light can pass through the diffusing layer with a lower probability of being scattered. Since the diffusing layer increases the probability of blue photons interacting with a phosphor material particle, less phosphor material can be used to generate a selected emission color. Such an arrangement can also increase luminous efficacy of the wavelength conversion component/device. Preferably the light diffractive material has an average particle size of less than about 150 nm where the excitation radiation comprises blue light. When the excitation radiation comprises UV light, the light diffractive material may have an average particle size of less than about 100 nm.

The light transmissive substrate can comprise any material that is substantially transmissive to visible light (380 nm to 740 nm) and typically comprises a polymer material such as a polycarbonate or an acrylic. Alternatively the substrate can comprise a glass.

The concept of a wavelength conversion component having a light diffusing layer composed of light diffractive particles that preferentially scatter light corresponding to wavelengths generated by the LEDs compared with light of wavelengths generated by the phosphor material is considered inventive in its own right. According to a further aspect of the invention a wavelength conversion component for a light emitting device comprising at least one blue light emitting solid-state light source, comprises a wavelength layer comprising particles of at least one phosphor material and a light diffusing layer comprising particles of a light diffractive material; wherein the light diffractive particle size is selected such that the particles will scatter excitation radiation relatively more than they will scatter light generated by the at least one phosphor material.

To increase the CRI (Color Rendering Index) of light generated by the device the device can further comprise at least one solid-state light source operable to generate red light.

Further details of aspects, objects, and advantages of the invention are described below in the detailed description, drawings, and claims. Both the foregoing general description and the following detailed description are exemplary and explanatory, and are not intended to be limiting as to the scope of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the present invention is better understood LED-based light emitting devices and phosphor wavelength conversion components in accordance with the invention will now be described, by way of example only, with reference to the accompanying drawings in which like reference numerals are used to denote like parts, and in which:

FIG. 1 shows schematic partial cutaway plan and sectional views of a solid-state light emitting device in accordance with an embodiment of the invention;

FIG. 2 is a schematic of a phosphor wavelength conversion component in accordance with an embodiment of the invention;

FIG. 3 is a schematic of a phosphor wavelength conversion component in accordance with another embodiment of the invention;

FIG. 11 is a schematic illustrating the principle of operation of a known light emitting device;

FIG. 12 is a schematic illustrating the principle of operation of the light emitting device having scattering particles mixed with phosphor particles in accordance with an embodiment of the invention;

FIG. 13 is a plot of emission intensity versus chromaticity CIE x for an LED-based light emitting device in accordance with the invention for different weight percent loadings of light reflective material;

FIG. 14 is a schematic illustrating a light emitting device having scattering particles within both a wavelength conversion layer and a diffusing layer according to an embodiment of the invention;

FIGS. 15 and 16 illustrate, respectively, a perspective view and a cross-sectional view of an application of a wavelength conversion component in accordance with some embodiments;

FIG. 17 is a schematic illustrating a light emitting device having a diffusing layer formed as a dome-shaped shell, in which a wavelength conversion layer forms an inner layer on an interior surface of the dome-shaped diffusing layer, according to an embodiment of the invention;

FIG. 18 is a schematic illustrating a light emitting device having a diffusing layer formed as a dome-shaped shell, in which a wavelength conversion layer substantially fills an interior volume formed by the interior surface of the dome-shaped diffusing layer, according to an embodiment of the invention;

FIG. 19 is a schematic illustrating a light emitting device having a diffusing layer formed as a dome-shaped shell, in which a wavelength conversion layer having scattering particles substantially fills an interior volume formed by the interior surface of the dome-shaped diffusing layer, according to an embodiment of the invention;

FIGS. 20A, 20B, and 20C illustrate an example of an application of a wavelength conversion component in accordance with some embodiments;

FIGS. 21A, 21B, and 21C illustrate another example of an application of a wavelength conversion component in accordance with some embodiments;

FIG. 22 illustrates another example of an application of a wavelength conversion component in accordance with some embodiments;

FIGS. 23A and 23B illustrate another example of an application of a wavelength conversion component in accordance with some embodiments; and

FIG. 24 illustrates a perspective of another application of a wavelength conversion component in accordance with some embodiments.

DETAILED DESCRIPTION

Some embodiments of the invention are directed to light emitting devices comprising one or more solid-state light emitters, typically LEDs, that is/are operable to generate excitation light (typically blue or UV) which is used to excite a wavelength conversion component containing particles of a photoluminescence materials (e.g. phosphor materials), such as a blue light excitable phosphor material or an UV excitable phosphor material. Additionally the wavelength conversion component comprises a light diffusing layer comprising particles of a light diffractive material (also referred to herein as “light scattering material”). One benefit of this arrangement is that by selecting an appropriate particle size and concentration per unit area of the light diffractive material, it is possible to make a device having an emission product color that is virtually uniform with emission angle over a ±60° range from the emission axis. Moreover the use of a light diffusing layer can substantially reduce the quantity of phosphor material required to generate a selected color of emitted light. In addition, the light diffusing layer can significantly improve the white appearance of the light emitting device in its OFF state.

For the purposes of illustration only, the following description is made with reference to photoluminescence material embodied specifically as phosphor materials. However, the invention is applicable to any type of any type of photoluminescence material, such as either phosphor materials or quantum dots. A quantum dot is a portion of matter (e.g. semiconductor) whose excitons are confined in all three spatial dimensions that may be excited by radiation energy to emit light of a particular wavelength or range of wavelengths. In addition, the following description is made with reference to radiation sources embodied specifically as blue light sources. However, the invention is applicable any type of radiation source, including blue light sources and UV light sources.

A solid-state light emitting device 10 in accordance with an embodiment of the invention will now be described with reference to FIG. 1 which shows schematic partial cutaway plan and sectional views of the device. The device 10 is configured to generate warm white light with a CCT (Correlated Color Temperature) of approximately 3000K and a luminous flux of approximately 1000 lumens.

The device 10 comprises a hollow cylindrical body 12 composed of a circular disc-shaped base 14, a hollow cylindrical wall portion 16 and a detachable annular top 18. To aid in the dissipation of heat the base 14 is preferably fabricated from aluminum, an alloy of aluminum or any material with a high thermal conductivity (preferably ≧200 Wm−1K−1) such as for example copper, a magnesium alloy or a metal loaded plastics material. For low cost production the wall 16 and top 18 are preferably fabricated from a thermoplastics material such as HDPP (High Density Polypropylene), nylon or PMA (polymethyl acrylate). Alternatively they can be fabricated from a thermally conductive material such as aluminum or an aluminum alloy. As indicated in FIG. 1 the base 14 can be attached to the wall portion 16 by screws or bolts 20 or by other fasteners or by means of an adhesive. As further shown in FIG. 1 the top 18 can be detachably mounted to the wall portion 16 using a bayonet-type mount in which radially extending tabs 22 engage in a corresponding annular groove in the top 18.

The device 10 further comprises a plurality (four in the example illustrated) of blue light emitting LEDs 24 (blue LEDs) that are mounted in thermal communication with a circular-shaped MCPCB (metal core printed circuit board) 26. The blue LEDs 24 can comprise 4.8 W Cetus™ C1109 chip on ceramic devices from Intematix Corporation of Fremont, Calif. in which each device comprises a ceramic packaged array of twelve 0.4 W GaN-based (gallium nitride-based) blue LED chips that are configured as a rectangular array 3 rows by 4 columns. Each blue LED 24 is operable to generate blue light 28 having a peak wavelength λ1 in a wavelength range 400 nm to 480 nm (typically 450 nm to 470 nm). As is known an MCPCB comprises a layered structure composed of a metal core base, typically aluminum, a thermally conductive/electrically insulating dielectric layer and a copper circuit layer for electrically connecting electrical components in a desired circuit configuration. The metal core base of the MCPCB 26 is mounted in thermal communication with the base 14 with the aid of a thermally conductive compound such as for example an adhesive containing a standard heat sink compound containing beryllium oxide or aluminum nitride. As shown in FIG. 1 the MCPCB can be attached to the base using screws or bolts 30.

To maximize the emission of light, the device 10 can further comprise light reflective surfaces 32, 34 that respectively cover the face of the MCPCB 26 and the inner curved surface of the top 18. Typically the light reflective surfaces 32, 34 can comprise a highly light reflective sheet material such as WhiteOptics™ “White 97” (A high-density polyethylene fiberbased composite film) from A.L.P. lighting Components, Inc of Niles, Ill., USA. As indicated in FIG. 1 a circular disc 32 of the material can used to cover the face of the MCPCB and a strip of the light reflective material configured as a cylindrical sleeve 34 that is inserted in the housing and is configured to cover the inner surface of the housing wall portion 16.

The device 10 further comprises a phosphor wavelength conversion component 36 that is operable to absorb a proportion of the blue light 28 (λ1) generated by the LEDs 24 and convert it to light 38 of a different wavelength (λ2) by a process of photoluminescence 36. The emission product 40 of the device 10 comprises the combined light of wavelengths λ1, λ2 generated by the LEDs 24 and the phosphor wavelength conversion component 36. The wavelength conversion component is positioned remotely to the LEDs 24 and is spatially separated from the LEDs a distance d that is typically at least 1 cm. In this patent specification “remotely” and “remote” means in a spaced or separated relationship. The wavelength conversion component 36 is configured to completely cover the housing 12 opening such that all light emitted by the lamp passes through the component 36. As shown the wavelength conversion component 36 can be detachably mounted to the top of the wall portion 16 using the top 18 enabling the component and emission color of the lamp to be readily changed.

As shown in FIG. 2, the wavelength conversion component 36 comprises, in order, a light transmissive substrate 42, a light diffusing layer 44 containing light diffractive particles and a wavelength conversion layer 46 containing one or more photoluminescent (e.g., phosphor) materials. As can be seen in FIG. 2 the wavelength conversion component 36 is configured such that in operation the wavelength conversion layer 46 faces the LEDs.

The light transmissive substrate 42 can be any material that is substantially transmissive to light in a wavelength range 380 nm to 740 nm and can comprise a light transmissive polymer such as a polycarbonate or acrylic or a glass such as a borosilicate glass. For the lamp 10 of FIG. 1 the substrate 42 comprises a planar circular disc of diameter (φ=62 mm and thickness t1 which is typically 0.5 mm to 3 mm. In other embodiments the substrate can comprise other geometries such as being convex or concave in form such as for example being dome shaped or cylindrical.

The diffusing layer 44 comprises a uniform thickness layer of particles of a light diffractive material, preferably titanium dioxide (TiO2). In alternative arrangements the light diffractive material can comprise barium sulfate (BaSO4), magnesium oxide (MgO), silicon dioxide (SiO2), aluminum oxide (Al2O3) or a powdered material with as high a reflectivity as possible, typically a reflectance of 0.9 or higher. The light diffractive material powder is thoroughly mixed in known proportions with a light transmissive liquid binder material to form a suspension and the resulting mixture deposited onto the face of the substrate 42 preferably by screen printing to form a uniform layer of thickness t2 (typically in a range 10 μm to 75 μm) that covers the entire face of the substrate. The quantity of light diffracting material per unit area in the light diffusing layer 44 will typically in a range 1 μg.cm−2 to 5 mg.cm−2.

Whilst screen printing is a preferred method for depositing the light diffractive diffusing layer 44, it can be deposited using other techniques such as for example slot die coating, spin coating, roller coating, drawdown coating or doctor blading. The binder material can comprise a curable liquid polymer such as a polymer resin, a monomer resin, an acrylic, an epoxy (polyepoxide), a silicone or a fluorinated polymer. It is important that the binder material is, in its cured state, substantially transmissive to all wavelengths of light generated by the phosphor material(s) and the LEDs 24 and preferably has a transmittance of at least 0.9 over the visible spectrum (380 nm to 800 nm). The binder material is preferably U.V. curable though it can be thermally curable, solvent based or a combination thereof. U.V. or thermally curable binders can be preferable because, unlike solvent-based materials, they do not “outgas” during polymerization. In one arrangement the average particle size of the light diffractive material is in a range 5 μm to 15 μm though as will be described it can be in a nanometer range (nm) and is advantageously in a range 100 nm to 150 nm. The weight percent loading of light diffractive material to liquid binder is typically in a range 7% to 35%.

The wavelength conversion layer 46 is deposited in direct contact with the light diffusing layer 44 without any intervening layers or air gaps. The phosphor material, which is in powder form, is thoroughly mixed in known proportions with a liquid light transmissive binder material to form a suspension and the resulting phosphor composition, “phosphor ink”, deposited directly onto the diffusing layer 44. The wavelength conversion layer is preferably deposited by screen printing though other deposition techniques such as slot die coating, spin coating or doctor blading can be used. To eliminate an optical interface between the wavelength conversion and diffusing layers 46, 44 and to maximize the transmission of light between layers, the same liquid binder material is preferably used to fabricate both layers; that is, a polymer resin, a monomer resin, an acrylic, an epoxy, a silicone or a fluorinated polymer.

The phosphor wavelength conversion layer 46 is preferably deposited by screen printing though other deposition techniques such as for example slot die coating, spin coating, roller coating, drawdown coating or doctor blading can be used. The binder material is preferably U.V. or thermally curable rather than being solvent-based. When a solvent evaporates the volume and viscosity of the composition will change and this can result in a higher concentration of phosphor material which will affect the emission product color of the device. With U.V. curable polymers, the viscosity and solids ratios are more stable during the deposition process with U.V. curing being used to polymerize and solidify the layer after deposition is completed. Moreover since in the case of screen printing of the phosphor ink multiple-pass printing may be required to achieve a required layer thickness, the use of a U.V. curable binder is preferred since each layer can be cured virtually immediately after printing prior to printing of the next layer.

The color of the emission product produced by the wavelength conversion component depends on the phosphor material composition and the quantity of phosphor material per unit area in the wavelength conversion layer 46. It will be appreciated that the quantity of phosphor material per unit area is dependent on the thickness t3 of the wavelength conversion layer 46 and the weight loading of phosphor material to binder in the phosphor ink. In applications in which the emission product is white or in applications in which the emission product has a high saturation color (i.e. the emission product comprises substantially all photoluminescence generated light) the quantity of phosphor material per unit area in the wavelength conversion layer 46 will typically be between 10 mg.cm−2 and 40 mg.cm−2. To enable printing of the wavelength conversion layer 46 in a minimum number of print passes the phosphor ink preferably has as high a solids loading of phosphor material to binder material as possible and preferably has a weight loading of phosphor material to binder in a range 40% to 75%. For weight loadings below about 40% it is found that five or more print passes may be necessary to achieve a required phosphor material per unit area. The phosphor material comprises particles with an average particle size of 10 μm to 20 μm and typically of order 15 μm.

In general lighting applications the emission product 40 will typically be white light and the phosphor material can comprise one or more blue light excitable phosphor materials that emit green (510 nm to 550 nm), yellow-green (550 nm to 570 nm), yellow (570 nm to 590 nm), orange (590 nm to 630 nm) or red (630 nm to 740 nm) light. The thickness t3 of the wavelength conversion layer, phosphor material composition and the density (weight loading) of phosphor material per unit area will determine the color of light emitted by the lamp.

The phosphor material can comprise an inorganic or organic phosphor such as for example silicate-based phosphor of a general composition A3Si(O,D)5 or A2Si(O,D)4 in which Si is silicon, O is oxygen, A comprises strontium (Sr), barium (Ba), magnesium (Mg) or calcium (Ca) and D comprises chlorine (Cl), fluorine (F), nitrogen (N) or sulfur (S). Examples of silicate-based phosphors are disclosed in U.S. Pat. No. 7,575,697 B2 “Silicate-based green phosphors”, U.S. Pat. No. 7,601,276 B2 “Two phase silicate-based yellow phosphors”, U.S. Pat. No. 7,655,156 B2 “Silicate-based orange phosphors” and U.S. Pat. No. 7,311,858 B2 “Silicate-based yellow-green phosphors”. The phosphor can also comprise an aluminate-based material such as is taught in co-pending patent application US2006/0158090 A1 “Novel aluminate-based green phosphors” and U.S. Pat. No. 7,390,437 B2 “Aluminate-based blue phosphors”, an aluminum-silicate phosphor as taught in co-pending application US2008/0111472 A1 “Aluminum-silicate orange-red phosphor” or a nitride-based red phosphor material such as is taught in co-pending United States patent application US2009/0283721 A1 “Nitride-based red phosphors” and International patent application WO2010/074963 A1 “Nitride-based red-emitting in RGB (red-green-blue) lighting systems”. It will be appreciated that the phosphor material is not limited to the examples described and can comprise any phosphor material including nitride and/or sulfate phosphor materials, oxy-nitrides and oxy-sulfate phosphors or garnet materials (YAG).

A further example of a phosphor wavelength conversion component 36 in accordance with the invention is illustrated in FIG. 3. In common with the wavelength conversion component of FIG. 2 the component comprises a light transmissive substrate 42, a light diffusing layer 44 and a wavelength conversion layer 46. In accordance with the invention the light diffusing and wavelength conversion layers 44, 46 are deposited in direct contact with one another. Again in operation the component is configured such that the wavelength conversion component is configured such that the light diffusing layer 44 faces the LEDs 24.

In operation blue light 28 generated by the LEDs 24 travels through the wavelength conversion layer 46 until it strikes a particle of phosphor material. It is believed that on average as little as 1 in 1000 interactions of a photon with a phosphor material particle results in absorption and generation of photo luminescence light 38. The majority, about 99.9%, of interactions of photons with a phosphor particle result in scattering of the photon. Due to the isotropic nature of the scattering process on average half of the photons will scattered in a direction back towards the LEDs. Tests indicate that typically about 10% of the total incident blue light 28 is scattered and emitted from the wavelength conversion component 36 in a direction back towards the LEDs. For a cool white light emitting device the amount of phosphor material is selected to allow approximately 10% of the total incident blue light to be emitted from the wavelength conversion component and contribute to the emission product 40 that is viewed by an observer 21. The majority, approximately 80%, of the incident light is absorbed by the phosphor material and re-emitted as photo luminescence light 38. Due to the isotropic nature of photo luminescence light generation, approximately half of the light 38 generated by the phosphor material will be emitted in a direction towards the LED. As a result only up to about 40% of the total incident light will be emitted as light 38 of wavelength λ2 and contributes to the emission product 38 with the remaining (up to about 40%) of the total incident light being emitted as light 38 of wavelength λ2 in a direction back towards the LED. Light emitted towards the LEDs from the wavelength conversion component 36 is re-directed by the light diffractive surfaces 32, 34 to contribute to the emission product and to increase the overall efficiency of the device.

One problem associated with a conventional LED lighting device that is addressed by embodiments of the invention is the non-white color appearance of the device in an OFF state. As discussed, during an ON state, the LED chip or die generates blue light and some portion of the blue light is thereafter absorbed by the phosphor(s) to re-emit yellow light (or a combination of green and red light, green and yellow light, green and orange or yellow and red light). The portion of the blue light generated by the LED that is not absorbed by the phosphor combined with the light emitted by the phosphor provides light which appears to the human eye as being nearly white in color.

However, in an OFF state, the LED chip or die does not generate any blue light. Instead, light that is produced by the remote phosphor lighting apparatus is based at least in part upon external light (e.g., sunlight or room lights) that excites the phosphor material in the wavelength conversion component, and which therefore generates a yellowish, yellow-orange or orange color in the photoluminescence light. Since the LED chip or die is not generating any blue light, this means that there will not be any residual blue light to combine with the yellow/orange light from the photoluminescence light of the wavelength conversion component to generate white appearing light. As a result, the lighting device will appear to be yellowish, yellow-orange or orange in color. This may be undesirable to the potential purchaser or customer that is seeking a white-appearing light.

According to some embodiments, the light diffusing layer 44 provides the additional benefit of addressing this problem by improving the visual appearance of the device in an OFF state to an observer 21. In part, this is because the light diffusing layer 44 includes particles of a light diffractive material that can substantially reduce the passage of external excitation light that would otherwise cause the wavelength conversion component to re-emit light of a wavelength having a yellowish/orange color.

The particles of a light diffractive material in the light diffusing layer 44 are selected, for example, to have a size range that increases its probability of scattering blue light, which means that less of the external blue light passes through the light diffusing layer 44 to excite the wavelength conversion layer 46. Therefore, the remote phosphor lighting apparatus will have more of a white appearance in an OFF state since the wavelength conversion component is emitting less yellow/red light.

The light diffractive particle size can be selected such that the particles will scatter blue light relatively at least twice as much as they will scatter light generated by the phosphor material. Such a light diffusing layer 44 ensures that during an OFF state, a higher proportion of the external blue light received by the device will be scattered and directed by the light diffractive material away from the wavelength conversion layer 46, decreasing the probability of externally originated photons interacting with a phosphor material particle and minimizing the generation of the yellowish/orange photoluminescent light. However, during an ON state, phosphor generated light caused by excitation light from the LED light source can nevertheless pass through the diffusing layer 44 with a lower probability of being scattered. Preferably, to enhance the white appearance of the lighting device in an OFF state, the light diffractive material within the light diffusing layer 44 is a “nano-particle” having an average particle size of less than about 150 nm. For light sources that emit lights having other colors, the nano-particle may correspond to other average sizes. For example, the light diffractive material within the light diffusing layer 44 for an UV light source may have an average particle size of less than about 100 nm.

Therefore, by appropriate selection of the average particle size of the light scattering material, it is possible to configure the light diffusing layer such that it scatters excitation light (e.g., blue light) more readily than other colors, namely green and red as emitted by the photoluminescence materials. FIG. 10 shows plots of relative light scattering versus TiO2 average particle size (nm) for red, green and blue light. As can be seen from FIG. 10, TiO2 particles with an average particle size of 100 nm to 150 nm are more than twice as likely to scatter blue light (450 nm to 480 nm) than they will scatter green light (510 nm to 550 nm) or red light (630 nm to 740 nm). For example TiO2 particles with an average particle size of 100 nm will scatter blue light nearly three times (2.9=0.97/0.33) more than it will scatter green or red light. For TiO2 particles with an average particle size of 200 nm these will scatter blue light over twice (2.3=1.6/0.7) as much as they will scatter green or red light. In accordance with some embodiments of the invention, the light diffractive particle size is preferably selected such that the particles will scatter blue light relatively at least twice as much as light generated by the phosphor material(s).

Another problem with remote phosphor devices that can be addressed by embodiments of the invention is the variation in color of emitted light with emission angle. In particular, remote phosphor devices are often subject to perceptible non-uniformity in color when viewed from different angles.

Embodiments of the invention correct for this problem, since the addition of a light diffusing layer 44 in direct contact with the wavelength conversion layer 46 significantly increases the uniformity of color of emitted light with emission angle θ. The emission angle θ is measured with respect to an emission axis 48 (FIG. 1). FIG. 4 shows plots of measured CIE color change versus emission angle θ for the lamp of FIG. 1 for wavelength conversion components 36 comprising diffusing layer 44 with percentage (%) weight loadings of light diffractive material to binder material of 0%, 7%, 12%, 16%, 23% and 35%, according to some example implementations of the invention. All emission color measurements were measured at a distance of 10 m from the lamp 10 for wavelength conversion components in which the light diffusing layer comprises light diffractive particles of TiO2 with an average particle size 25 μm. For comparison the data for a 0% percentage loading of TiO2 correspond to a wavelength conversion component that does not include a light diffusing layer.

where CIE xθ° is the measured CIE chromaticity x value at an emission angle of θ°, CIE x0° is the measured CIE chromaticity x value for an emission angle of θ=0°, CIE yθ° is the measured CIE chromaticity y value at an emission angle of θ° and CIE y0° is the measured CIE chromaticity y value at an emission angle of θ=0°. It will be appreciated that the CIE change is normalized to the light color at an emission angle θ=0° (i.e. the CIE change is always 0 for θ=0°).

As can be seen in FIG. 4 for a wavelength conversion component with no light diffusing layer (i.e. 0% Ti02 loading), the color of light generated by such a lamp can alter by a CIE change of nearly 0.07 for emission angles up to θ=60°. In comparison for a wavelength conversion component 36 in accordance with the invention that includes a light diffusing layer 44 with a percentage weight loading of TiO2 of only 7% the change in emission color over a 60° range drops to about 0.045. As can be seen from this figure, increasing the percentage weight loading of TiO2 decreases the change in emission color over a 60° angular range. For example for a 35% TiO2 percentage weight loading the CIE color change is less than 0.001. Although the change in emission color with emission angle decreases with increasing TiO2 loading the total emission intensity will also decrease.

FIG. 5 shows measured luminous efficacy versus CIE color change at an emission angle θ=60° for wavelength conversion components 36 comprising diffusing layer 44 with percentage (%) weight loadings of TiO2 to binder material of 0%, 7%, 12%, 16%, 23% and 35% for an example implementation of the invention. The luminous efficacy values are normalized relative to a lamp that does not include a light diffusing layer (i.e. 0% TiO2 loading). The CIE color change @ θ=60° is determined from the relationship:

where CIE x60° is the measured CIE chromaticity x value at an emission angle of 60°, CIE x0° is the measured CIE chromaticity x value for an emission angle of 0°, CIE y60° is the measured CIE chromaticity y value at an emission angle of 60° and CIE y0° is the measured CIE chromaticity y value at an emission angle of 0°. As can be seen from FIG. 5, there can be as much as a 25% decrease in luminous efficacy for a wavelength conversion component with a light diffusing layer containing a 35% weight loading of TiO2. It will be appreciated when selecting the weight loading of light diffractive material in light diffusing layer a balance should be struck between improving emission color uniformity with emission angle and the decrease in luminous efficacy of the lamp. Wavelength conversion component in accordance with some embodiments of the invention preferably has a light diffusing layer with percentage weight loading of light diffractive material to binder material in a range 10% to 20%.

FIG. 8 is a 1931 chromaticity diagram showing the color CIE x, CIE y) of emitted light at emission angles θ=0°, 15°, 30°, 45° and 60° for a an example 3000K white light emitting LED-based lamp in accordance with the invention for wavelength conversion components containing 0%, 10%, 15% and 20% weight loadings of TiO2. For comparison FIG. 8 also includes the black body radiation curve and ANSI C78.377A “Specification for chromaticity of white solid state lighting products” S and R quadrangles for white light of 3500K and 3000K respectively. Each quadrangle is equivalent to approximately seven MacAdam ellipses whilst each sub quadrangle (S02, S03, S06, S07, R02, R03, R06, R07) is equivalent to approximately four McAdam ellipses. As is known a MacAdam ellipse is a region on a chromaticity diagram which contains all colors which are indistinguishable, to the average human eye 21, from the color at the center of the ellipse. As can be seen from FIG. 8 for a lamp without a light diffusing layer (0% TiO2), the variation in emission color for emission angles of over a range θ=0° to 60° is approximately three MacAdam ellipses. For a lamp including a light diffusing layer with a 10% weight loading of TiO2, the variation in emission color with emission angle is less than two MacAdam ellipses with a corresponding decrease in luminous efficacy of about 2% (FIG. 7). For a lamp including a light diffusing layer with a 15% weight loading of TiO2, the variation in emission color with emission angle is approximately one MacAdam ellipse with a corresponding decrease in luminous efficacy of about 5% (FIG. 7). For such a lamp, an average person 21 would be unable to perceive the variation in emission color with emission angle. For a lamp including a light diffusing layer with a 20% weight loading of TiO2 the variation in emission color with emission angle is less than one MacAdam ellipse with a corresponding decrease in luminous efficacy of about 9% (FIG. 7). It will be appreciated the inclusion of a light diffusing layer 44 in accordance with the invention can virtually eliminate the effects of emission color variation with emission angle whilst maintaining an acceptable luminous efficacy.

Embodiments of the present invention can also be used to reduce the amount of phosphor materials that is required to manufacture an LED lighting product, thereby reducing the cost of manufacturing such products given the relatively costly nature of the phosphor materials. In particular, the addition of a light diffusing layer 44 composed of particles of a light diffractive material can substantially reduce the quantity of phosphor material required to generate a selected color of emitted light. This means that relatively less phosphor is required to manufacture a wavelength conversion component as compared to comparable prior art approaches. As a result, it will be much less costly to manufacture lighting apparatuses that employ such wavelength conversion components, particularly for remote phosphor lighting devices.

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